The terms and concepts presented in this appendix should prove helpful in understanding the OSPF domain discovered and monitored by Network Protocol Manager for OSPF.
Begin by examining Interior gateway protocol and exterior gateway protocol links:
The links between the routers within an independent network, or autonomous system (AS), are referred to as Interior Gateway Protocol (IGP) links. The links between routers in different autonomous systems are referred to as Exterior Gateway Protocol (EGP) links. OSPF is an IGP.
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ABR
Area Border Router. A router that has interfaces in multiple OSPF areas. It runs a copy of the basic OSPF algorithm for each area to which it belongs.
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ABR System Redundancy Group
Two or more ABRs that interface with and back up the same OSPF areas.
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AS
Autonomous System. A collection of networks, or more precisely, the routers joining those networks, that are under the same administrative authority and that share a common routing strategy.
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ASBR
Autonomous System Border Router. An OSPF router that exchanges routing information with routers that are running OSPF or other interior gateway protocols and belong to other autonomous systems.
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BDR
Backup Designated Router. An OSPF router elected on a broadcast (multi-access) network segment to back-up the Designated Router.
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DR
Designated Router. An OSPF router elected on a broadcast (multi-access) network segment to flood routing updates. Instead of each router exchanging OSPF updates with every other router on the broadcast segment, each non-DR router on the segment builds a full adjacency with the DR to exchange information with the DR, and the DR relays the information to the other non-DR routers.
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EGP
Exterior Gateway Protocol. A routing protocol, such as Border Gateway Protocol (BGP), used to exchange routing information between two routers in a network of autonomous systems. An EGP protocol maintains routes between autonomous systems.
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IGP
Interior Gateway Protocol. A routing protocol used to calculate routes and exchange routing information among routers within an autonomous system.
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LSA
Link-state advertisement. When a network link changes state (up to down, or vice versa) in an OSPF network, the change is flooded throughout the network as a link-state advertisement. All the routers note the change and recompute their routes accordingly.
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MTU
Maximum transmission unit. A setting that controls the maximum IP packet size that a PC will send.
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Multi-access segment
A network segment supporting three or more routers. A network segment is part of an Ethernet or other network on which all message traffic is common to all nodes, that is, a message is broadcast from one node on the segment and received by all others on the segment.
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NBMA
Non-Broadcast Multiple Access. A network without broadcast capabilities, but where all interfaces on the network are fully meshed (connected); for example, a fully meshed Frame Relay cloud.
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OSPF
Open Shortest Path First. A link-state based interior gateway protocol, which is defined in RFC 1253, that is used to distribute routing information within an autonomous system. Each OSPF router generates a full data model of its OSPF area (portion of the autonomous system), including all IP subnets and all routers in the area, and executes a shortest path first (SPF) algorithm to determine the best route to each IP subnet in the data model.
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OSPF adjacency
OSPF adjacency, also known as OSPF neighbor relationship, is the next step after OSPF routers become neighbors. In order to minimize the amount of information exchanged on a particular broadcast (multi-access) network segment, OSPF elects one router to be a Designated Router (DR), and one router to be a Backup Designated Router (BDR), on each broadcast segment. Routers that become adjacent have the same link-state database.
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OSPF area
A set of IP subnets within an autonomous system. An OSPF area limits the flooding of link-state updates on an OSPF router to just those changes within its area. If more than one OSPF area is configured for an autonomous system, one of the areas is designated the backbone area (Area 0). All non-backbone areas within an autonomous system have a physical connection to the backbone area.
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OSPF area type
The OSPF area’s support for importing autonomous system (AS) external link-state advertisements (LSAs). An OSPF area can be configured in one of five ways, based on what routes external to the area are advertised into the area by the Area Border Routers (ABRs) and the Autonomous System Border Routers (ASBRs) within the area.
Normal Area: All OSPF routes, whether from outside the area or outside the AS, are advertised into this area. The backbone area is always a normal area.
Stub Area: No routes that are external to the AS are advertised into this area. Practically, this means that all packets to addresses outside the AS are sent to the nearest ABR for routing by the backbone area.
Totally Stub Area: No routes external to this area or the AS are advertised into this area except for the default router (0.0.0.0). All packets destined for addresses outside this area or the AS go to the nearest ABR for routing by the backbone.
Not-So-Stubby Area (NSSA): No routes external to the AS are advertised into this area except for the external routes advertised by an ASBR within this area. All packets destined for addresses outside the AS go to the nearest ABR, unless the addresses can be reached by an ASBR within this area.
Totally NSSA: No routes external to this area or the AS are advertised into this area except for the default router (0.0.0.0) and the external routes advertised by an ASBR within this area. All packets destined for addresses outside this area or the AS go to the nearest ABR, unless the addresses are AS external addresses that can be reached by an ASBR within this area.
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OSPF interface
A link between an OSPF router and a network. The state of an OSPF interface is a description of the interface and its relationship to its neighboring routers. One or more router adjacencies may develop over an interface.
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OSPF neighbors
OSPF routers that share a common network segment become neighbors on that segment. Neighbors are elected through the Hello protocol. Two-way communication exists between each pair of neighbors.
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OSPF neighbor relationship
See OSPF adjacency in this list.
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OSPF network
Interconnected routers that are running OSPF services. An OSPF network can exist for four network types: broadcast, non-broadcast multiple access (NBMA), point-to-point, and point-to-multipoint.
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OSPF router
A router that is running an OSPF service.
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OSPF service
An OSPF process: an instance of the OSPF routing protocol that is running in memory.
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OSPF virtual link
A tunnel that connects an OSPF area to the backbone area (Area 0) for an autonomous system. OSPF virtual links are common when merging OSPF networks from two companies.
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VLAN
Virtual Local Area Network. A network of computers that behave as if they are connected to the same wire, even though they may actually be physically located on different network segments of a LAN. VLANs are configured through software rather than hardware, which makes them extremely flexible.
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VPN
Virtual Private Network. A private-access network over public connections.
